a) Field the Invention
The invention concerns processing aids for peeling LPB electrolytes. More particularly, the invention relates to a method for peeling LPB amorphous electrolytes from their coating substrates or protective films.
b) Description of Prior Art
Lithium/polymer electrolyte batteries (hereinafter designated LPB) are manufactured by laminating three main types of films: a film of a positive electrode containing an electrochemically active material such as vanadium oxide, a film of electrolyte made of a polymer and a salt of lithium, and a film of lithium. Each of these films has a thickness between 15 and 50 .mu.m, for a total thickness of elementary film of the battery of 100 to 150 .mu.m. About 30 meters of elementary film, 15 cm wide, are typically required to give a battery of 100 Wh.
The films of electrolyte are prepared, for example, by coating a solution of polymer and lithium salt on a plastic substrate of the polypropylene type, and evaporating the solvent in a drying tunnel. A protective film of polyethylene is then placed on the electrolyte, before winding it into a roll, in order to prevent the electrolyte from adhering to the back of the polypropylene substrate. A roll of positive electrode is prepared separately by solvent coating, with a polyethylene protective film also applied, then the two rolls are brought to a laminator. The polyethylene protective films are peeled off and the electrolyte is laminated to the positive electrode. The polypropylene substrate is then peeled off from the electrolyte/positive electrode laminate, and a film of lithium is laminated on top of the electrolyte to give a complete LPB cell.
With crystalline electrolytes, made for example of polyethylene oxide and non-plasticizing lithium salts, such as lithium perchlorate or lithium trifluorosulfonate, peeling is rather easy. However, with highly amorphous electrolytes, containing for example copolymers or highly plasticizing salts of the TFSI type (lithium trifluorosulfonimide), it is practically impossible to peel the polypropylene substrate without pulling off some of the electrolyte, because of the very adhesive character of the latter.
Substrates less adhesive and less polar than polypropylene, such as FEP (fluorinated ethylene propylene copolymer) or polyethylene (PE), cannot be used, because either they are not wetted properly by the electrolyte solutions (in the case of FEP), or they will stretch or deform in the drying tunnel (PE), or they are still too adhesive to be peeled off without damage to the electrolyte.
Internal additives known to facilitate peeling, such as silicone derivatives, also cannot be used in the electrolytes, because of their detrimental effect on the electrochemical properties of the cell.
Cooling the electrolyte films before peeling, at temperatures lower than -60.degree. C. (the glass transition temperature of the electrolytes), somewhat facilitates peeling, but not enough to enable a very uniform peeling on the entire surface and at a speed acceptable for industrial applications.